Iran's interior ministry said Thursday there would be no live debates in the run-up to next month's presidential election.

"Based on a decision by the Election Campaign Monitoring Commission, the election debates of the candidates will be broadcast pre-recorded," said ministry spokesman Seyed Salman Samani.

The commission also released guidelines for the debates, telling candidates they are not allowed to "blacken the image of the country... or the actions of the executive, administrative, legislative or judicial bodies".

Live debates have been a hugely popular feature of the last two elections in 2009 and 2013, with many analysts saying President Hassan Rouhani's performance was key to his surprise victory last time.

The decision was criticised by one conservative candidate, Alireza Zakani, who said it would "seriously damage transparency".

The decision appeared to come as a surprise to state broadcaster IRIB, whose deputy head Morteza Mir Bagheri had earlier described the live debates as "the most popular programmes" on the station, and the "best way for people to get to know the candidates".

The heated TV debates in 2009 between the then hardline president Mahmoud Ahmadinejad and his reformist opponents fired up the electorate and were seen as partly responsible for the post-election unrest that occurred when Ahmadinejad was re-elected amid allegations of vote-rigging.

More than 1,600 people have signed up to run in the election on May 19.

That number will be whittled down by the conservative-dominated Guardian Council in the coming days, with around half-a-dozen candidates normally permitted to stand.